Polyol-based method for producing ultra-fine silver powders

ABSTRACT

The present invention provides a metallic composition, which contains a plurality of ultra-fine metallic particles (e.g., ultra-fine copper, nickel, or silver particles) having at least one desirable feature, such as, tight size distribution, low degree of agglomeration, and high degree of crystallinity and oxidation resistance. The present invention further provides a method for forming the ultra-fine metallic particles. Also provided are a substance or substrate coated with the ultra-fine metallic particles and a method of coating a substance or substrate with the ultra-fine metallic particles. Furthermore, the present invention provides a method of controlling the size of ultra-fine metal particles formed in a reducing reaction in a liquid. Also provided is a method for producing ultra-fine metallic particles, which utilizes a concentrated reaction system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.nonprovisional application Ser. No. (not yet assigned), filed Oct. 29,2004, and is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to ultra-fine metalliccompositions and methods of making thereof. The present inventionfurther relates to methods of depositing ultra-fine metalliccompositions onto various substrates.

BACKGROUND OF THE INVENTION

Ultra-fine metallic particles have many unique physical and chemicalcharacteristics, which make them ideal materials for a variety ofapplications, such as electronics, catalysis, metallurgy, anddecorative. Compared to the various particle-producing techniques usedin the art, the methods based on the chemical precipitation in solutionsprovide several advantages, e.g., low manufacturing cost and a very goodcontrol of the mechanism of metal particles formation. Others in the arthave successfully prepared micron and submicron-size metallic powders ofCo, Cu, Ni, Pb, and Ag using chemical-based techniques, such as the onesbased on the reduction in alcohols or polyols. For example, U.S. Pat.No. 4,539,041 discusses a method for producing micrometer-size metallicparticles by using polyols to convert various metallic compounds intometal powders.

These procedures, however, are characterized by rather lowconcentrations of metallic precursors and consume large quantities oforganic solvents per unit weight of metallic powder produced.Furthermore, the metallic powders produced using these procedures have awide size distribution, a low degree of crystallinity, and in the caseof the base metals, a pronounced tendency to oxidation.

SUMMARY OF THE INVENTION

The present invention provides a metallic composition, which includes aplurality of ultra-fine metallic particles (e.g., ultra-fine copper,nickel, or silver particles) having at least one desirable feature, suchas tight size distribution, low degree of agglomeration, and high degreeof crystallinity and oxidation resistance.

In one aspect, the present invention provides a method for formingcompositions having a plurality of ultra-fine metallic particles (e.g.,ultra-fine copper, nickel, or silver particles), and the metalliccomposition produced therewith, where the plurality of ultra-finemetallic particles may be obtained in accordance with a process thatincludes the steps of:

(a) forming a reaction mixture containing a precursor of a metal, abranched dispersing agent, and an alcoholic agent;

(b) adjusting the temperature of the reaction mixture to a temperaturesuitable for reducing the metal precursor to the metallic state (“thereaction temperature”);

(c) maintaining the reaction mixture under the reaction temperature fora time sufficient to reduce the precursor of the metal to metalparticles; and optionally,

(d) isolating the metal particles.

In one embodiment, the branched dispersing agent may be a branchedpolyol, e.g. pentaerythritol. In another embodiment, the reactionmixture further may contain at least one other dispersing agent, such aslinear polyols (e.g., sorbitol and/or mannitol) and ammonium or sodiumsalts of polynaphtalene sulphonic/formaldehyde co-polymers. In yetanother embodiment, the alcoholic agent may be 1,2-propylene glycol,1,3-propylene glycol, diethyleneglycol, or the combinations thereof. Instill another embodiment, the method of the present invention mayfurther include a step of adjusting the pH of the reaction mixture(e.g., by introducing a buffering agent, such as, triethanolamine).

In another aspect, the present invention provides a substance orsubstrate coated with a plurality of ultra-fine metallic particles(e.g., ultra-fine copper, nickel, or silver particles) having at leastone desirable feature, such as tight size distribution, low degree ofagglomeration, and high degree of crystallinity and oxidationresistance.

Also provided is a method of coating a substance with a plurality ofultra-fine metallic particles (e.g., copper, nickel, or silverparticles), and the coated substance produced therewith, including thesteps of:

(a) forming a reaction mixture containing the substance, a precursor ofa metal, a branched dispersing agent, and an alcoholic agent;

(b) adjusting the temperature of the reaction mixture to a temperaturesuitable for reducing the precursor of the metal to metal particles(“the reaction temperature”);

(c) maintaining the reaction mixture under the reaction temperature fora time sufficient to reduce the precursor of the metal to metalparticles and permit the resulting metal particles to form a coating onthe surface of the substance; and optionally,

(d) isolating the coated substance.

In one embodiment, the branched dispersing agent may be a branchedpolyol, e.g. pentaerythritol. In another embodiment, the reactionmixture may further contain at least one other dispersing agent, such aslinear polyols (e.g., sorbitol and/or mannitol) and ammonium or sodiumsalts of polynaphtalene sulphonic/formaldehyde co-polymers. In yetanother embodiment, the alcoholic agent may be 1,2-propylene glycol,1,3-propylene glycol, diethyleneglycol, or the combinations thereof. Instill another embodiment, the method of the present invention mayfurther include a step of adjusting the pH of the reaction mixture(e.g., by introducing a buffering agent, such as, triethanolamine).

In yet another aspect, the present invention provides a method ofcontrolling the size of ultra-fine metal particles (e.g., copper,nickel, or silver particles) formed in a reducing reaction in a liquid,where the method includes the step of adjusting the pH of the liquid,e.g. by introducing a buffering agent into the liquid, such astriethanolamine. In one embodiment, the ultra-fine metal particles maybe formed by reducing a precursor of the metal in the liquid containinga polyol composition. In another embodiment, the polyol composition maycontain a branched dispersing agent, such as a branched polyol (e.g.,pentaerythritol). Further provided is a method for producing ultra-finemetallic particles, which utilizes a concentrated reaction system.

Additional aspects of the present invention will be apparent in view ofthe description that follows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 includes images that depict the effects of buffering agent TEA onthe copper particles produced by the method in accordance with oneembodiment of the present invention, where the reaction mixture includes50% 1,2-PG, 50-x% DEG, and x% TEA and (a) x=0; (b) x=1.5; (c) x=5; and(d) x=10. The images were acquired using field emission scanningelectron microscope.

FIG. 2 shows the effects of buffering agent TEA on the size of thecopper particles produced by the method according to one embodiment ofthe present invention.

FIG. 3 illustrates the effects of various mixtures of polyols on thesize of the copper particles produced by the method according to oneembodiment of the present invention, where the reaction mixture includes(a) 1,2-PG and TEA (90:10 v/v); (b) 1,2-PG, 1,3-PG, and TEA (50:40:10,v/v, respectively); and (c) 1,2-PG, DEG, and TEA (50:40:10, v/v,respectively). Images were acquired using a scanning electron microscopeat two magnifications (5,000× and 10,000×).

FIG. 4 demonstrates the effects of changing the concentration of thecopper salt on the size of the copper particles produced by the methodaccording to one embodiment of the present invention, where the reactionmixture includes: (a) 0.174 g/cm³ CuCO₃; (b) 0.261 g/cm³ CuCO₃; (c)0.348 g/cm³ CuCO₃; and (d) 0.400 g/cm³ CuCO₃. Images were acquired usinga scanning electron microscope (5000× magnification).

FIG. 5 contains the typical XRD pattern of highly crystalline copperparticles produced by the method according to one embodiment of thepresent invention, displaying a pronounced split of the (220), (311),and (222) reflections.

FIG. 6 shows the SEM images of nickel particles produced by the methodaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein and in the appended claims, the singular forms “a,” “an,”and “the” include plural references unless the content clearly dictatesotherwise. Thus, for example, reference to “a particle” includes aplurality of such particles, and reference to “the polyol” is areference to one or more polyols and equivalents thereof known to thoseskilled in the art, and so forth. All publications, patent applications,patents, and other references mentioned herein are incorporated byreference in their entirety.

The present invention provides ultra-fine metallic particles having atleast one desirable feature, such as, a tight size distribution, a highdegree of crystallinity, oxidation resistance, and a low degree ofagglomeration, or a combination thereof. The present invention alsogenerally provides a more cost effective chemical based method forproducing metallic powders than those known in the art. The presentinvention further generally provides a method for producing metallicpowders having ultra-fine metallic particles of a particular size byreducing precursors of the metals in an alcoholic agent at higherconcentrations than those used in the systems known in the art toproduce particles with substantially the same sizes. The concentratedmethod or system of the present invention may therefore be used toreduce the cost of making ultra-fine metallic particles in terms ofenergy, resources, waste, etc.

In at least one embodiment of the invention, the present method orsystem beneficially produces metallic powders that include a pluralityof ultra-fine metallic particles having at least one desirable feature,e.g., a tight size distribution, a high degree of crystallinity,oxidation resistance, and a low degree of agglomeration, or acombination thereof. As used herein and in the appended claims, the term“ultra-fine particles” generally includes particles having diameters ofabout 1 nm-10 μm, preferably, about 10-5,000 nm, and more preferably,50-3,000 nm, and even more preferably, 100-1000 nm. The ultra-finemetallic particles may be the metallic particles of various metals,including, without limitation, transitional metals and noble metals,such as Ag, Au, Co, Cr, Cu, Fe, In, Ir, Mn, Mo, Ni, Nb, Os, Pd, Pt, Re,Rh, Ru, Sn, Ta, Ti, V, W, Zn, and the combinations thereof. In oneembodiment, the metallic powders include a metal selected from the groupconsisting of Cu, Ni, and Ag.

Unlike other metallic powders appearing in the art, in one embodiment,the system of the present invention produces metallic powders thatinclude ultra-fine metallic particles, particularly, isometricultra-fine metallic particles, that have a tight size distribution. Thebreadth of the size distribution, as used herein, generally refers tothe degree of variation in the diameter of the ultra-fine metallicparticles in a metallic composition. Tight, used in this context,indicates a relatively small variation in the size of the ultra-fineparticles. In one embodiment, the ultra-fine metallic particles aredeemed to have a tight size distribution when the diameters of at leastabout 80%, preferably, at least about 85%, and more preferably, at leastabout 95%, of the ultra-fine metallic particles of the present inventionare within the range of N±15% N, where N is the average diameters of theultra-fine metallic particles. The diameters of the ultra-fine metallicparticles may be measured by a number of techniques, such as, by anelectron microscope, particularly, a scanning electron microscope (e.g.field emission scanning electron microscope).

The metallic powders produced with the system of the present inventionmay also include ultra-fine metallic particles that have a low degree ofagglomeration. The degree of agglomeration may be expressed using theindex of agglomeration I_(agg1), which is the ratio between the averagesize distribution of the ultra-fine metallic particles (“PSD50%”) andthe average diameter of the particles. The average particle sizedistribution may be determined by any methods known in the art,including, but not limited to, dynamic light scattering (DLS), laserdiffraction, and sedimentation methods, while the average particle sizemay be determined by averaging the diameter of the individual ultra-finemetallic particles obtained by, e.g., electron microscopy. An I_(agg1)value of 1.0 indicates completely lack of agglomeration, while anincrease in I_(agg1) value indicates an increase in the degree ofaggregation. In one embodiment, the powders of ultra-fine metallicparticles of the present invention have an I_(agg1) value of 1.2 orless.

The metallic powders produced in accordance with the present inventionmay also include ultra-fine metallic particles that have a high degreeof crystallinity. The term “degree of crystallinity,” as used herein andin the appended claims, generally refers to the ratio between the sizeof the crystallites in the metallic powder and the diameter of themetallic particles. The size of the constituent crystallites may bededuced from XRD measurements using the Sherrer's equation, while theparticle size may be determined by electron microscopy. A larger ratioof the size of the crystallites in comparison to the diameter of themetallic particles indicates an increased degree of crystallinity and alower internal grain boundary surface. In one embodiment, the ultra-finemetallic particles have a high degree of crystallinity if at least about80%, preferably, at least about 85%, more preferably, at least about90-95%, and even more preferably, about 100% of the ultra-fine metallicparticles of the present invention are highly crystalline. The highdegree of crystallinity is reflected by the visible splitting of thepeaks corresponding to the (220), (311), and (222) reflections in theXRD spectrum (see, e.g. FIG. 5).

The metallic powders produced in accordance with the present inventionmay also include ultra-fine metallic particles that are resistant tooxidation. In one embodiment, the ultra-fine metallic particles of thepresent invention undergo minimal or insubstantial oxidation whenexposed to the air in ambient environment for about 12 months or longer.Oxidation is generally minimal or insubstantial if the ultra-finemetallic particles display an increase of less than about 5-10% in theiroxygen content as measured by the LECO combustion method. In anotherembodiment, the ultra-fine metallic particles of the present inventiondo not undergo substantial oxidation when exposed to a temperature of upto 100° C. in ambient environment for about 120 minutes. In stillanother embodiment, the overall weight gain of the ultra-fine metallicparticles is minimal or insubstantial when they are heated in the air at20° C./minute up to about 200-220° C. and does not exceed about 80% ofthe theoretical weight gain when the temperature reaches about 8000 C.For example, the theoretical weight gain of 100 g Cu particles whentreated under the above condition is ˜26 g. The weight gain of 100 gultra-fine Cu particles of the present invention when treated under thesame condition does not exceed ˜21 g.

The present invention also provides methods for producing metallicpowders, and also metallic powders produced therewith, that include aplurality of ultra-fine metallic particles that, in one embodiment, areobtained by: (a) forming a reaction mixture containing a precursor of ametal, a branched dispersing agent, and an alcoholic agent; (b)adjusting the temperature of the reaction mixture to a temperaturesuitable for reducing the precursor of the metal to metal particles(“the reaction temperature”); (c) maintaining the reaction mixture underthe reaction temperature for a time effective to reduce the precursor ofthe metal to metal particles; and optionally, (d) isolating the metalparticles. In one embodiment, the method of the present inventionfurther includes a step of adjusting the pH of the reaction mixture(e.g., by introducing a buffering agent, such as, triethanolamine).

The process of the present invention may be used to manufactureultra-fine particles of various metals, such as Ag, Au, Co, Cr, Cu, Fe,In, Ir, Mo, Ni, Nb, Os, Pd, Pt, Re, Rh, Ru, Sn, Ta, Ti, V, and W, andalloys or composites containing these metals. A metal precursor is mixedwith an alcoholic composition or agent, which converts the metalprecursor to ultra-fine metal particles under various reactionconditions. The term “alcoholic composition” or “alcoholic agent,” asused herein and in the appended claims, generally includes alcohols,such as, monohydroxylic and polyhydroxylic alcohols (polyols).

The form of the metal precursor used in the reaction depends upon theparticular metal itself and the types of ultra-fine metal particleproducts desired. Generally, the precursor may be any metal-containingcompound or complex that may be reduced into elemental metal under thereaction conditions. The precursor needs not to be completely soluble inthe reaction mixture. Typical precursors include, e.g., metal carbonatesand hydrates thereof, metal acetates and hydrates thereof, metalchlorides and hydrates thereof, metal nitrates, metal oxides, metaloxalates, metal hydroxides, and acids including the desired metal aspart of an oxyanion (e.g., tungstic acid) and salts of such acids (e.g.,sodium tungstate and potassium hexachlorplatinate). In one embodiment,metal carbonates, such as CuCO₃, NiCO₃, CoCO₃, Ag₂CO₃, may be used asthe precursor for producing ultra-fine metal particles of Cu, Ni, Co,and Ag, respectively. The use of metal carbonates may be a criticalelement in providing highly dispersed metallic particles at high metalion concentrations, as the carbonate counter ions may decompose andleave the system. Consequently, the ionic strength of the reactionsystem does not increase substantially during the reaction, whichpromotes the stabilization of the metallic particle dispersion. Inanother embodiment, a mixture of metal precursors, such as, metalcarbonates and metal acetates or metal salycilates may be used for theproduction of ultra-fine metallic particles. The inventors have alsofound that the presence of organic counter ions, such as acetate andsalycilate, may further enhance the stability of the metallic particlesat high concentrations of metal ions. In yet another embodiment, agentswhich provide organic counter ions, such as acetate ions or salycilateions, may be administered into the reaction system of the presentinvention.

The term “branched dispersing agent” as used herein and in the appendedclaims includes any dispersing agents, which have at least one sidegroup that includes at least one carbon, such as, a branched polyol. Theterm “branched polyol” as used herein and in the appended claimsincludes any polyols, which have at least one side group that includesat least one carbon. Branched polyols suitable for the process of thepresent application includes, without limitation, 2-C-methylerythritol,2-C-methylthreitol, and pentaerythritol (“PE”). Branched polyols mayhave a number of roles in the reaction mixture, including functioning asa dispersant and/or a reducing agent. The term “reducing agent” as usedherein and in the appended claims generally includes any agent which iscapable of reducing a precursor of a metal to elemental metals and/ormetal particles. The inventors of the present invention discovered that,compared to the use of linear polyols, using branched polyols, mixturesof branched and linear polyols, and mixtures of branched polyols andammonium or sodium salts of polynaphtalene sulphonic/formaldehydeco-polymers in accordance with the present invention, offers someunexpected advantages, e.g., resulting in metallic particles withtighter size distribution, lower degree of agglomeration, high degree ofcrystallinity, and less susceptible to oxidation, etc. The term “linearpolyols” includes, without limitation, molecules containing linearchains of 3 to 7 carbon atoms, where each carbon atom having a hydroxylgroup attached, such as, sorbitol and mannitol. In one embodiment, thebranched polyol may be PE. In another embodiment, the sodium salts ofpolynaphtalene sulphonic/formaldehyde co-polymers may be the Daxaddispersants, such as, Daxad 11G and Daxad 19.

The polyol composition used in the process of the present invention maybe commanded by the particular reaction. A broad range of polyols may beused in the process, such as the polyols disclosed in U.S. Pat. Nos.4,539,041 and 5,759,230, each of which are hereby incorporated herein byreference. The polyols may be in either liquid or solid form. In oneembodiment, 1,2-propylene glycol (“1,2-PG”), 1,3-propylene glycol(“1,3-PG”), diethyleneglycol (“DEG”), or the combinations thereof, maybe used in the reaction mixture. In another embodiment, a mixture of1,2-PG and DEG may be used as the reducing polyol.

When forming the reaction mixture, the branched dispersing agent (e.g.the branched polyol) and the alcoholic agent may be either unheated orheated. Generally, the reaction temperature may be maintained oradjusted to about 80-350° C., or more preferably, about 110-200° C. Forexample, to produce ultra-fine Cu particles, 1,2-PG, DEG, and PE may bemixed and heated to bring the temperature of the mixture to about 70° C.The required amount of CuCO₃ may then be added into the polyol mixtureat about 80-85° C. after PE is fully dissolved. The reaction mixture mayfurther be heated to bring the temperature of the mixture to anappropriate reaction temperature. In the present example, the reactiontemperature is about 180-185° C.

The resulting ultra-fine metal particles may be obtained followingstandard protocols known in the art, such as by precipitation,filtration, and centrifugation. The particles may further be washed,such as by using methanol or ethanol, and dried, such as by air, N₂, orvacuum.

The size and the uniformity of the ultra-fine metallic particles may beaffected by a variety of factors, such as the types of metal precursor,branched polyol, alcoholic agent, and dispersant used, the concentrationof the metal ions, the reaction temperature, and the pH of the reactionmixture. In one embodiment, the pH of the reaction mixture may beadjusted to control the size of the ultra-fine metallic particlesproduced at any given metal precursor concentrations. The inventorsdiscovered that pH changes significantly affect the reduction reactionand the formation of metallic particles. In a preferred embodiment, thepH of the reaction mixture may be adjusted by adding a buffering agent.The term “buffering agent” as used herein generally includes an agentwhich, upon addition to the reaction mixture, reduces the change of pHof the reaction mixture caused by the H⁺ produced during the reaction orwhen an acid or base is added into the reaction mixture. The bufferingagent is added to the reaction mixture to control, e.g., increase,decrease, or stabilize, the pH of the reaction mixture in order tocontrol the size of the particles produced by the reaction system at agiven concentration of a metal precursor in the reaction mixture. Inthis respect, the pH of the reaction mixture may be controlled toproduce smaller particles than would otherwise be possible at aparticular concentration of the metal precursor. Examples of bufferingagents are triethanolamine (“TEA”),4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (“HEPES”),4-morpholinepropanesulfonic acid (“MOPS”),tris(hydroxymethyl)aminomethane (“Tris”), andN-[tris(hydroxymethyl)methyl]-2-aminoethanesulfonic acid (“TES”). In oneembodiment, the buffering agent may be TEA.

The inventors discovered that the size of ultra-fine metallic particlesformed by the process of the present invention may be significantlyaffected by the amount of buffering agent added to the reaction mixture.For example, in a typical reaction system with 1,2-PG (250 ml), DEG (250ml), and CuCO₃ (200 g), the pH of the reaction mixture measured at roomtemperature in the absence of PE decreases from about 8.6 at thebeginning of the process to about 4.85 at the end of the reaction andthe average size of the copper particles product is about 2.4 μm. Theaddition of 2% TEA (final concentration) raised the final pH to about6.20 and the size of the copper decreased to about 1.5 μm. When 5% and10% of TEA (final concentration) was introduced into the reactionmixture, the pH at the end of the reaction was about 7.70 and about8.60, respectively, and the size of the copper particles produced by theprocess was reduced to about 700 nm and about 300 nm, respectively.

Moreover, controlling of the pH of the reaction mixture during thereduction process offers additional unexpected benefits. For example, itmay dramatically reduce the cost of making ultra-fine metallic particlesby enabling the uses of a concentrated reaction system. The polyol-basedsystems known in the art are rather diluted systems, i.e. theconcentrations of the metal ions in these systems are kept low in orderto form ultra-fine metal particles in the sub-mircrometer scale,typically lower than 5-10%. Therefore, the diluted systems will consumemore energy and materials (polyols, etc.) to produce a particular sizeof ultra-fine metallic particles than using a concentrated system.Furthermore, the concentrated system of the present invention reducesthe cost of processing the organic solvent waste. In the polyol systemof the present invention, the pH of the reaction mixture may becontrolled, e.g., by the addition of a buffering agent, such as TEA.Thus, the reaction rate is not or significantly less affected by thepotential change in the pH as a result of the presence of a largequantity of metal precursors in the system. For example, Cu particleswith a size of about 300 nanometer may be produced by adding more than200 g of CuCO₃ into a reaction mixture of 500 ml (250 ml 1,2-PG, 200 mlDEG, and 50 ml TEA) following the process of the present invention,while Cu particles with a much larger size (about 2.4 μm) are formedwhen the same amount of CuCO₃ is added into a reaction mixture where thepH is not controlled (250 ml 1,2-PG and 250 ml DEG).

The inventors also discovered that the types of polyol used in theprocess affect the size and uniformity of the metallic particlesproduced. For example, in a typical reaction, the ultra-fine copperparticles formed in reaction mixture of 1,2-PG as the sole reducingpolyol shown the widest particle size distribution (100-700 nm). Theuniformity of the copper particles considerably improves when polyolmixtures, such as a mixture of 1,2-PG and 1,3-PG or a mixture of 1,2-PGand DEG, are used (see, e.g., FIG. 3). Furthermore, comparing to the useof 1,3-PG, the use of DEG resulted in somewhat larger copper particles(e.g., 300 nm vs. 500 nm, respectively). The inventors furtherdemonstrated that the copper particles produced by the process, whichutilized a mixture of 1,2-PG and DEG, has the highest uniformity (i.e.the tightest size distribution) (see, e.g., FIG. 3).

The present invention further provides a substrate coated with aplurality of ultra-fine metallic particles, where the plurality ofultra-fine metallic particles have at least one desirable feature, suchas, a tight size distribution, a low degree of agglomeration, a highdegree of crystallinity, and oxidation resistance. The term “substrate”as used herein includes, without limitation, metallic subjects (e.g.,metallic particles, flakes, tubes, and sheets), plastic materials,ceramic subjects, fibers, films, glasses, polymers, organic materials(e.g. resins), inorganic materials (e.g., carbon nanotubes), and anyother object capable of being coated with the ultra-fine metallicparticles produced in accordance with the present invention. Theultra-fine metallic particles may be the metallic particles of variousmetals, preferably, Cu, Ni, and Ag.

In one aspect, the present invention provides a method of coating asubstrate with a plurality of ultra-fine metallic particles, and alsocoated substrates produced therewith, by: (a) forming a reaction mixturecontaining the substrate, a precursor of a metal, a branched dispersingagent (e.g., a branched polyol), and an alcoholic agent; (b) adjustingthe temperature of the reaction mixture to a temperature suitable forreducing the precursor of the metal to metal particles (“the reactiontemperature”); (c) maintaining the reaction mixture under the reactiontemperature for a time effective to reduce the precursor of the metal tometal particles and permit the resulting metal particles to form acoating on the surface of the substance; and optionally, (d) isolatingthe coated substance. In one embodiment, the ultra-fine metallicparticles may be introduced to the surface of the substrate in such amanner that they form a uniform and continuous layer(s) the surface.

EXAMPLES

The following examples illustrate the present invention, which are setforth to aid in the understanding of the invention, and should not beconstrued to limit in any way the scope of the invention as defined inthe claims which follow thereafter.

Example 1 Materials

The copper carbonate (CuCO₃) was supplied by Shepherd Chemical Co.1,2-PG and DEG were obtained from Alfa Aesar (Ward Hill, Mass.). 1,3-PGand PE were obtained from Avocado Research Chemical Ltd., while TEA waspurchased from Aldrich (Milwaukee, Wis.).

Example 2 Copper Particles Synthesis

All experiments were carried out in a 1 L-4-necked round flask equippedwith a Dean Stark trap and a refluxing condenser. The stirring wasprovided by a two inch Teflon-blade connected to a variable speed mixer.The amount of cupric carbonate used in the precipitation process was ingeneral kept at 200 g (1.62 mol) although smaller or larger amounts wereoccasionally used as well (i.e., 87 g and 300 g). The CuCO₃ was addedinto 500 cm³ polyols or polyols mixture containing 15 g PE (for 200 gCuCO₃). The dispersant agent (PE) was initially added in polyols andheated at low power (10%) in the heating mantle to bring the temperatureto 70° C. The required amounts of CuCO₃ were added into the flask at80-85° C. after the PE was fully dissolved. The CuCO₃/polyol mixture wasstirred at 500 RPM in all experiments. The mixture was then heated at50% setting of heating power until the temperature reaches 180-185° C.The copper particles obtained were washed three times with ethanol(3×400 mL) and were filtered using a vacuum system and Whatmann #50filter paper. The particles were then dried overnight at 80° C. in aregular oven.

Example 3 Particles Characterization

The morphology of copper particles was investigated by scanning electronmicroscopy (SEM) using a JEOL-JSM 6300 scanning microscope at 15 kVaccelerating voltage and the magnification between 2500 and 10000. Also,the copper powders were analyzed by field emission scanning electronmicroscopy (FE-SEM) with 5 kV accelerating voltage and the same range ofmagnification using a JEOL JSM-7400F field emission scanning electronmicroscope.

Discussed below are results obtained by the inventors in connection withthe experiments of Example 1-3:

In order to evaluate the effect of pH in the formation of Cu particlesin polyols, variable amounts of triethanolamine (TEA) were added intothe dispersion of CuCO₃ prior to the heating as shown in Table 1. Thereaction time in the presence of TEA varied between 3 and 4 hours, theaddition of more base tending to speed up the reaction. The images ofcopper particles produced in the manner described in the presentinvention, obtained by FE-SEM, are illustrated in FIG. 1.

Almost all copper particles prepared by the reduction of coppercarbonate in polyols or mixtures of polyols in the presence of TEA areisometric and very crystalline in shape. Their diameter can be variedfrom several hundred nanometers (200-300 nm) to several micrometers (2-3μm) by modifying the amount of TEA (pH) added into the reaction mixture.

The experimental conditions and data including the size range of thecopper particles obtained at different pH values are summarized inTable 1. In all experiments containing TEA, the copper particlesretained the original morphology obtained in the absence of TEA(pH=4-5).

The particles sizes shown in Table 1 were obtained by averaging the sizeof minimum 50 particles generated in each experiment. TABLE 1Experimental condition and characteristics of the copper powder obtainedin polyols mixtures containing TEA Polyols Average (ml) TEA particlesize 1,2-PG DEG ml (%) CuCO₃ (g)¹ PE (g) pH² (μm) 500 0 0 0 200 15 n/a2.2 250 250 0 0 200 15 4.85 2.5 250 240 10 2 200 15 6.20 1.2 250 225 255 200 15 7.70 0.7 250 200 50 10 200 15 8.62 0.3¹Lot# 1018121.²The pH of emulsions was measured at room temperature at the end of thereaction.

The changes in the average diameter of copper particles size produced asa function of the concentration of TEA are illustrated in FIG. 2. Thedifferences in average diameter of particles obtained in similarexperimental conditions between different lots of CuCO₃ are 10%.

In order to evaluate the influence of different polyol composition inthe preparation of copper particles, several experiments were carriedout using pure 1,2-PG and mixtures of 1,2-PG containing DEG and 1,3-PGrespectively. In all these experiments a 10% content of TEA and the sameamounts of PE (7 g) and CuCO₃ (87 g) were used. FIG. 3 a, b, c shows theSEM images at two magnifications (5000× and 10000×) of the copperparticles formed in a 1,2-PG : DEG : TEA =50:40:10 (v/v) mixture and1,2-PG: 1,3-PG : TEA=50:40:10 (v/v), respectively. For comparison, FIG.3 includes also the SEM of copper particles obtained in 1,2-PG :TEA=90:10 (v/v).

For copper particles formed in 1,2-PG/TEA mixture the SEM analysis shownthe widest particle size distribution (100-700 nm). The uniformity ofthe copper particles improves when polyol mixtures were used. It appearsthat the nature of the second polyol affects the size of the particles,the addition of DEG generating larger particles (0.5 μm) than in thecase of 1,3-PG (0.3 μm). Furthermore, the results of this set ofexperiments tend to suggest that the addition of DEG leads to the mostuniform copper particles.

It has been shown in the inventors' earlier work that, when the amountof the CuCO₃ is changed, the size of the copper particles decreases withthe decrease in the concentration of the Cu ions of the system. Thistrend causes an increase in the cost of producing ultra-fine Cuparticles with a decreased size. It is expected that, in moreconcentrated systems, the pH of the reaction mixture decreases more,causing a decrease in the reducing power of the polyol and a slowdown inthe reaction rate of the second stage of the copper reduction (Cu⁺→Cu⁰).The inventors demonstrated that the fine copper particles could be infact produced even in highly concentrated system providing that the pHof the reaction mixture is controlled. In order to evaluate theinfluence of CuCO₃ concentration on the copper particles size, asystematic study was carried out using 87 g (0.174 g/cm³); 130.5 g(0.261 g/cm³); 174 g (0.348 g/cm³); and 200 g (0.40 g/cm³) CuCO₃ inreduction process. For all experiments a fixed amount of PE (7 g) and afixed 1,2-PG: DEG: TEA ratio (250:200:50, v/v) were used. The pH of theinitial slurry did not change with the amount of carbonate used and itdecreased only slightly during the reduction process. The SEM picturesat 5000× magnification of copper particles obtained at different CuCO₃concentrations are illustrated in FIG. 4.

The average size copper particles was ˜0.5 μm for all the CuCO₃concentrations tested, the differences between separate precipitationsbeing less than ±20%. A somehow better homogeneity was observed at thelowest concentration, probably because of the higher dipersant:metalratio.

These results further confirm the hypothesis that the rate of thereduction with polyols is pH dependent and that by controlling the pHduring the reaction the size of the resulting Cu particles can also becontrolled. The discovery provided by the present invention may havesignificant implications in the production of ultra-fine Cu powderssince it enables a manufacturing method which may be easily scaled up toproduce ultra-fine Cu powder at very competitive prices.

Among the factors that may affect the size of copper particles producedby the chemical reduction of copper carbonate in polyols and/or polyolsmixtures, one of the most influencing factor is the pH of solution. Theinventors demonstrated that this parameter can be adjusted by adding TEAin controlled concentrations. At high pH values (8.6-9.0), such as when10% TEA was added into the reaction mixture, smaller copper particles(size range 0.2-0.5 μm) are formed, while the sizes of copper particlesincrease with the decreasing of pH. The size of the copper particles isnot substantially affected by the changes of pH when the value of the pHof the reaction mixture is less than 5.75 or higher than 8.5.

The second factor that influences the copper particle size is thecomposition of the polyol mixtures used in the precipitation process.When the copper particles were synthesized in only one polyol (e.g.,1,2-PG), a broad size distribution was obtained (1.5-2.6 μm). Theuniformity of copper particles obtained in polyol mixtures is somehowimproved compared to the case when pure 1,2-PG is used, the narrowestdistribution being obtained in 1,2-PG: DEG mixtures (2-2.8 μm) (FIG. 3).

The third factor that influences the size of copper particles is theCuCO₃ concentration. When the pH of the system is not controlled, thediameters of the Cu particles varies significantly with theconcentrations of the Cu precursor. However, when the pH is controlledby adding a buffering agent (i.e., TEA), the size of the copperparticles was relatively stable for a wide range of CuCO₃ (0.174-0.40g/cm³). An approximately 10% increasing in average diameter of copperparticles obtained was observed in experiments using a higher CuCO₃concentration (0.40 g/cm³) (FIG. 4).

Example 4 Preparation of Ultra-Fine Nickel Particles

Nickel carbonate (NiCO₃) was supplied by Shepherd Chemical Co., 1,2-PGand DEG were obtained from Alfa Aesar (Ward Hill, Mass.). 1,3-PG and PEwere obtained from Avocado Research Chemical Ltd., and the PalladiumChloride solution (PdCl₂) was obtained from OMG (South Plainfield,N.J.).

All experiments were carried out in a 1 L-4-necked flask equipped with arefluxing condenser preceded by a dean stark trap and 7″ extension. Thestirring was provided by a two inch Teflon-blade connected to a variablespeed mixer. The amount of nickel carbonate used in the precipitationprocess was in general kept at 140 g (1.18 mol). NiCO₃ was added into a500 ml polyol mixture, composed of 50% PG and 50% DEG and 7 g PE. Thedispersing agent, PE, was added in the polyol and heated at 75% power inthe heating mantel to bring the temperature up to 70° C. The requiredamount of NiCO₃ was then added into the flask at 80-85° C., after the PEwas fully dissolved. The NiCO₃/polyol mixture was stirred at 500 RPM inall experiments. The mixture was continually heated at 75% power untilthe suspension reached the end point. The nickel particles shown in FIG.6 were washed three times with ethanol (3×400 ml) and were filtered witha vacuum system using Whatman #50 filter paper. The particles were thendried overnight at 100° C. in a regular oven.

Example 5 Preparation of Ultra-Fine Silver Particles

All experiments were carried out in a 1 L-4-necked flask equipped with arefluxing condenser preceded by a dean stark trap and 7″ extension. Thestirring was provided by a two inch Teflon-blade connected to a variablespeed mixer. The amount of silver carbonate used in the precipitationprocess was in general kept at 100 g. Ag₂CO₃ was added into a 500 mlpolyol mixture, composed of 50% PG and 50% DEG and 7 g PE. Thedispersing agent was initially added in the polyol and mixed untilcompletely dissolved. The required amount of Ag₂CO₃ was then added intothe flask, after the PE was fully dissolved. The carbonate/polyolmixture was stirred at 500 RPM in all experiments. The mixture wascontinually heated until the suspension reached the end point.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be appreciated by oneskilled in the art, from a reading of the disclosure, that variouschanges in form and detail can be made without departing from the truescope of the invention in the appended claims.

1. A metallic composition comprising a plurality of ultra-fine silverparticles, wherein the plurality of ultra-fine silver particles isresistant to oxidation.
 2. The metallic composition of claim 1, whereinthe plurality of ultra-fine silver particles undergoes minimal oxidationfor 12 months in ambient environment, wherein oxidation is minimal whenthe oxygen content of the ultra-fine silver particles is less than about5-10% at the end of such period of time.
 3. The metallic composition ofclaim 1, wherein the plurality of ultra-fine silver particles undergoesminimal oxidation when the plurality of ultra-fine silver particles isexposed to a temperature up to 100° C. for 120 minutes in air.
 4. Themetallic composition of claim 1, wherein the plurality of ultra-finesilver particles undergoes minimal oxidation when the plurality ofultra-fine silver particles is heated in air at 20° C./minute up to200-220° C.
 5. The metallic composition of claim 1, wherein oxidation ischaracterized by a weight gain in the plurality of ultra-fine silverparticles and wherein the weight gain of the plurality of ultra-finesilver particles does not exceed about 80% of a theoretical weight gainfor the plurality of ultra-fine silver particles when the plurality ofultra-fine silver particles is heated in air at 20° C./minute to 800° C.6. The metallic composition of claim 1, wherein the plurality ofultra-fine silver particles has a tight size distribution.
 7. Themetallic composition of claim 6, wherein the plurality of ultra-finesilver particles has a tight size distribution when at least about 80%of the plurality of ultra-fine silver particles has a diameter withinthe range of N±15% N, wherein N is an average diameter of the pluralityof ultra-fine silver particles.
 8. The metallic composition of claim 1,wherein the plurality of ultra-fine silver particles has a high degreeof crystallinity.
 9. The metallic composition of claim 8, wherein atleast about 80-95% of the plurality of ultra-fine silver particle ishighly crystalline.
 10. The metallic composition of claim 1, wherein theplurality of ultra-fine silver particles has a low degree ofagglomeration.
 11. The metallic composition of claim 10, wherein thedegree of agglomeration is measured with an I_(agg) value and whereinthe I_(agg1) of the plurality of ultra-fine silver particles is lessthan about 1.2.
 12. A metallic composition comprising a plurality ofultra-fine silver particles, wherein the plurality of ultra-fine silverparticles is obtained in accordance with the process comprising thesteps of: (a) forming a reaction mixture comprising a precursor ofsilver, a branched dispersing agent, and an alcoholic agent; (b)adjusting the temperature of the reaction mixture to a reactiontemperature suitable for reducing the precursor of silver to silverparticles; (c) maintaining the reaction mixture under the reactiontemperature for a time sufficient to reduce the precursor of silver tosilver particles; and optionally, (d) isolating the silver particles.13. The metallic composition of claim 12, wherein the brancheddispersing agent is a branched polyol.
 14. The metallic composition ofclaim 13, wherein the branched polyol is pentaerythritol.
 15. Themetallic composition of claim 12, wherein the reaction mixture furthercomprises at least one other dispersant selected from the groupconsisting of a linear polyol dispersant and a salt of polynaphtalenesulphonic/formaldehyde co-polymer.
 16. The metallic composition of claim12, wherein the alcoholic agent is at least one polyol selected from thegroup consisting of 1,2-propylene glycol, 1,3-propylene glycol, anddiethyleneglycol.
 17. The metallic composition of claim 16, wherein thealcoholic agent is the mixture of 1,2-propylene glycol anddiethyleneglycol.
 18. The metallic composition of claim 12, wherein theprecursor of silver is silver carbonate.
 19. The metallic composition ofclaim 12, wherein the precursor of silver is a mixture of silvercarbonate and at least one of silver acetate and silver salycilate. 20.The metallic composition of claim 12, wherein the reaction temperatureis about 180-185° C.
 21. The metallic composition of claim 12, whereinthe process further comprises adjusting pH of the reaction mixture. 22.The metallic composition of claim 21, wherein the pH of the reactionmixture is adjusted by introducing a buffering agent into the reactionmixture.
 23. The metallic composition of claim 22, wherein the bufferingagent is triethanolamine.
 24. The metallic composition of claim 12,wherein the reaction mixture further comprises an agent which releasesan organic counter ion.
 25. The metallic composition of claim 24,wherein the organic counter ion is at least one of an acetate and asalycilate.
 26. A method for forming a plurality of ultra-fine silverparticles comprising the steps of: (a) forming a reaction mixturecomprising a precursor of silver, a branched dispersing agent, and analcoholic agent; (b) adjusting the temperature of the reaction mixtureto a reaction temperature suitable for reducing the precursor of silverto silver particles; (c) maintaining the reaction mixture under thereaction temperature for a time sufficient to reduce the precursor ofsilver to silver particles; and optionally, (d) isolating the silverparticles.
 27. The method of claim 26, wherein the branched dispersingagent is a branched polyol.
 28. The method of claim 27, wherein thebranched polyol is pentaerythritol.
 29. The method of claim 26, whereinthe reaction mixture further comprises at least one other dispersantselected from the group consisting of a linear polyol dispersant and asalt of polynaphtalene sulphonic/formaldehyde co-polymer.
 30. The methodof claim 26, wherein the alcoholic agent is at least one polyol selectedfrom the group consisting of 1,2-propylene glycol, 1,3-propylene glycol,and diethyleneglycol.
 31. The method of claim 30, wherein the alcoholicagent is the mixture of 1,2-propylene glycol and diethyleneglycol. 32.The method of claim 26, wherein-the precursor of silver is silvercarbonate.
 33. The method of claim 26, wherein the precursor of silveris a mixture of silver carbonate and at least one of silver acetate andsilver salycilate.
 34. The method of claim 26, wherein the reactiontemperature is about 180-185° C.
 35. The method of claim 26, furthercomprising adjusting pH of the reaction mixture.
 36. The method of claim26, wherein the pH of the reaction mixture is adjusted by introducing abuffering agent into the reaction mixture.
 37. The method of claim 36,wherein the buffering agent is triethanolamine.
 38. The method of claim26, wherein the reaction mixture further comprises an agent whichreleases an organic counter ion.
 39. The method of claim 38, wherein theorganic counter ion is at least one of an acetate and a salycilate.